DC to AC power converters are known which use two switching devices connected in series in each leg of the converter, the junction of each series-connected pair of switching devices comprising an output of the converter. Such a converter has been used in adjustable speed ac motor drives where a pulse width modulated three-phase bridge inverter supplies a voltage of adjustable amplitude and frequency to a motor.
The use of a self-extinguishing power semiconductor device such as a gate turn-off thyristor (GTO) or a high power transistor in a power converter requires both a series inductive snubber to limit the rate of change of current with respect to time (di/dt) when turning on and a shunt capacitive snubber to limit the rate of change of voltage with respect to time (dv/dt) when turning off. After switching (either the turning on or turning off of either switching device of the leg), energy is trapped in the snubber elements. In the prior art, this energy is usually discharged through resistors. It is also known to recover some of the snubber energy, as described in a paper by A. Ferraro, "An Overview of Low Loss Snubber Technology for Transistor Converters", Conference Record IEEE Power Electronics Specialists Conference, 1982, pages 466-477.
It is also known to recover trapped snubber energy via a single transformer such that the transformer is loaded only during commutation transients. This transformer can be relatively small and can employ an iron core so that the two windings can be reasonably well coupled, thus improving the efficiency of energy recovery. However, in the known configurations of the recovery transformer, the reset time, which is the time it takes the flux in the transformer to reduce to zero, is so long as to impose an unacceptable upper limit on the frequency at which the transformer can operate. If the transformer is not reset by the next commutation, it will saturate. As a result, the snubber energy will remain trapped as a high magnetizing current circulating through the snubber inductance and the saturated primary winding of the transformer until it is dissipated by losses. Thus, little if any energy would be recovered.